1. Field of the Invention
The present invention relates to a method of manufacturing a wiring board to be suitably used for manufacturing a multilayer wiring board with an inner via hole connection.
2. Description of the Related Art
In recent years, an increase in the density of a wiring board has been required with a reduction in the size and weight of electronic equipment. Correspondingly, the multilayer structure of a wiring layer has been enhanced. As such a structure of the multilayer wiring board, generally, an insulating layer and a wiring layer having a pattern formed thereon are sequentially laminated and the wiring layer is conductively connected through an inner via hole.
A method of manufacturing a multilayer wiring board with the inner via hole connection has been described in Japanese Unexamined Patent Publication No. 6-268345, for example. This method includes the steps of forming a through hole on a prepreg having a releasing film on at least one of the surfaces of the prepreg, filling the through hole with a conductive paste, peeling the releasing film, laminating a metal foil on a surface from which the releasing film is peeled, and heating and pressurizing the laminated product.
As is clearly described in the publication, the prepreg to be used in the method is usually obtained by impregnating a heat-resistant nonwoven fabric such as a polyamide fiber with a thermosetting epoxy resin. In some cases, moreover, a glass fiber impregnated with the thermosetting epoxy resin is used.
If the prepreg obtained by impregnating the heat-resistant nonwoven fabric or the glass fiber with the thermosetting epoxy resin is used, however, the workability of the through hole is apt to be deteriorated by a fiber constituting a nonwoven fabric. Moreover, it is difficult to produce thin and uniform woven and nonwoven fabrics even if the thickness of an insulating layer is to be reduced in order to enhance the workability of a laser via process. Thus, a reduction in the thickness of a layer has been limited.
As described in Japanese Unexamined Patent Publication No. 11-68275, furthermore, the prepreg using a woven fabric or a nonwoven fabric has a problem in that the conductive filler of a conductive paste is apt to flow to the outer peripheral portion of a via hole during heating and pressurizing, resulting in a deterioration in the reliability of a conductive connection and the generation of a short circuit between via holes.
Therefore, it is an object of the present invention to provide a wiring board which is excellent in the workability of a via hole and enhances the reliability of a conductive connection between wiring layers, and a method of manufacturing the same.
In order to achieve the object, the present inventor vigorously studied the structure, material and thickness of a prepreg and then found that the object can be achieved by using a porous film having a specific structure for the reinforcing phase of the prepreg, thereby finishing the present invention.
More specifically, the present invention provides a method of manufacturing a wiring board, comprising the steps of:
forming a through hole on a prepreg including a releasing resin film on at least one of its surfaces, the prepreg being obtained by impregnating a porous film having a thickness of 5 to 90 xcexcm and a porosity of 30 to 98% with a half cured thermosetting resin;
filling said through hole with a conductive paste containing a conductive filler;
peeling said releasing resin film;
laminating a metal foil on a surface from which said releasing resin film is peeled; and
heating and pressurizing the laminated product.
According to the method of manufacturing a wiring board in accordance with the present invention, the workability of the through hole can be more enhanced by using the porous film having a specific thickness and porosity for the reinforcing phase of the prepreg as compared with the case in which a woven fabric or a nonwoven fabric is used for the reinforcing phase. Moreover, the porous film has a three-dimensional bone. Therefore, as compared with the woven fabric or the nonwoven fabric having a one-dimensional fiber aggregate, the conductive filler can be trapped more easily and hardly flows to the outer peripheral portion of the via hole during heating and pressurization. For this reason, the reliability of a conductive connection can be enhanced and a short circuit between the via holes can be prevented more easily. This respect is also apparent from the result of an embodiment. As a result, it is possible to provide a method of manufacturing a wiring board which is excellent in the workability of the via hole and enhances the reliability of a conductive connection between wiring layers.
It is preferable that the porous film has a sponge structure and a mean pore diameter in a section thereof is smaller than the mean particle diameter of the conductive filler. Consequently, the pore structure is comparatively uniform in a portion having the sponge structure and the mean pore diameter in the section is smaller than the mean particle diameter of the conductive filler. Therefore, a leakage from the via hole of the conductive filler can be prevented more reliably so that the reliability of the conductive connection can be enhanced more reliably and the short circuit between the via holes can be prevented. The mean pore diameter in the section is obtained by calculating the diameter of a pore appearing on the whole section of the porous film based on a circular conversion with a number average for the section and is specifically obtained by a measuring method according to the xe2x80x9cExamplexe2x80x9d.
On the other hand, the present invention provides a wiring board comprising an insulating layer obtained by impregnating a porous film having a thickness of 5 to 90 xcexcm and a porosity of 30 to 98% with a thermosetting resin and curing them, and a conductive connection structure between wiring layers in which a through hole provided on the insulating layer is filled with a conductive paste,
wherein the conductive connection structure has a conductive filler at only a boundary surface with said porous film and in an inner part thereof. According to the wiring board of the present invention, the through hole of the insulating layer is filled with the conductive paste so that the conductive filler is present at only the boundary surface with the porous film and in the inner portion thereof. Therefore, the density of the conductive filler is maintained so that the reliability of the conductive connection can be enhanced. Moreover, a workability can also be enhanced during the formation of the through hole.